Disconnectable buoy

Abstract

The invention relates to floating mooring construction (1) comprising a vessel (2) having a hull (35) with a cavity (9) for receiving a mooring buoy (8) carrying at least one flowline (13), and a pulling device (22, 24) connected to the mooring buoy for pulling the buoy towards the vessel. A flexible element (27) is comprised between a wall of the recess (9) and the buoy. The invention is characterised in that the vessel comprises a turret (5) around which the vessel can weathervane, the recess (9) being provided in the lower part of the turret (5), wherein the buoy (8) comprises near its lower end an annular groove (26) in which the flexible element (27) is seated, the outer surface of the flexible element including a sharp angle (&agr;) with a center line (21) of the buoy. The buoy is attachable to the turret by the pulling device (22, 24) pulling the buoy into the cavity (9) and compressing the flexible element (27) against the wall of the cavity (9). The present invention provides a semi-permanent system, which is able to transmit relatively large mooring forces to the turret.

Description

[0001] The invention relates to a floating mooring construction comprising a vessel having a hull and having a cavity for receiving a mooring buoy, the mooring buoy carrying at least one flowline and anchor lines connected to the buoy via chain guides, and a pulling device connected to the mooring buoy for pulling the buoy towards the vessel, a flexible element being comprised between a wall of the recess and the buoy.

[0002] Such a mooring construction for buoy loading of hydrocarbons at sea is known from U.S. Pat. No. 5,651,708. In the known construction the buoy comprises an outer body which is attached to the recess of the vessel by means of horizontally movable locking wedges. The outer body of the buoy is at its bottom provided with a combined fender and watertight packing which abuts the lower circumferential portion of the recess, forming a sealing force transmitting area between the outer body and the hull of the vessel. The inner part of the buoy is rotatably seated in the fixed outer body. Upon severe weather conditions or emergency situations, the buoy can be released from the vessel by the detaching of the pull in hawser and by attaching a float to the pull in hawser of the buoy. The buoy may be reattached by taking in the pull hawser by a winch on the vessel. In the known system, relatively large forces are transferred to the bearings between the outer buoy body and the rotatable inner part. Therefore, the buoy will need to be released from the vessel at predetermined wave bights. This may reduce useful operating time of the vessel after disconnecting of the buoy.

[0003] The known mooring buoy is only suitable for the relatively small mooring forces and is relatively small as it is intended to be used with a light mooring system and is not suitable for deep water. The known mooring system can only carry a limited number of risers. The bearing system is relatively light, watertight and hence complex and not easily accessible for repair and change out purposes.

[0004] It is an object of the present invention to provide a mooring system which is semi-permanent and which provides a reliable connection between the buoy and the vessel, also during extreme weather conditions. It is further object of the present invention to provide a semi-permanent mooring system, which can during specific severe weather conditions, or emergencies be decoupled from the vessel, and relatively easily reattached.

[0005] It is a further object of the present invention to provide a system which is able to transfer relatively large forces from the mooring buoy to the vessel and around which the vessel can freely weathervane.

[0006] It is again an object of the invention to provide a mooring system which is suitable for use in deep water and which can carry a large number of risers. The buoy should be easily accessible for repair or change-out.

[0007] Thereto, the mooring construction according to the present invention is characterised in that the vessel comprises a turret around which the vessel can weathervane, the recess being provided in the lower part of the turret, wherein the buoy or the recess comprises near its lower end the flexible element, the outer surface of which includes a sharp angle with a centre line of the buoy, the buoy being attachable to the turret by the pulling device pulling the buoy into the cavity and compressing the flexible element against the wall of the recess, the lower end of the buoy being located at or in the proximity of the lower end of the recess, the anchor lines being with their upper ends fixed to the buoy.

[0008] By providing the recess in the turret around which the vessel can weathervane, the buoy need not be provided with rotatable parts. Hence, larger forces can be taken up by the buoy.

[0009] By compression of the flexible element when the buoy is wedged into the recess by the pulling device, such as a pull in hawser or chain, a firm connection is possible. According to the present invention, a pretension is applied to the buoy by wedging it into the recess such that the flexible element is deformed and any play is removed, whereafter the buoy can be locked in place for instance by a hydraulic locking mechanism which is known per se. Large radial mooring forces are transmitted from the buoy to the hull via the flexible elements.

[0010] In the mooring arrangement according to the present invention, the anchor lines are connected directly to the buoy and are not tensioned from the vessel itself as is the case in prior art mooring devices. Hereby a tensioned mooring line configuration is obtained that is able to absorb relatively large forces, using only a single pull in hawser for the buoy.

[0011] Preferably the flexible element is seated in an annular groove on the recess or on the buoy. By adjusting the angle of the flexible element with respect to the vertical centre line of the buoy, the stroke of the pulling device for wedging the buoy into the recess can be easily adjusted and the stiffness of the connection of the buoy to the turret can be varied.

[0012] In an embodiment of the present invention, the diameter of the recess and of the buoy near their lower ends is larger than the diameter of the upper part of the turret. By providing a large diameter buoy which is substantially completely seated within the recess, large forces can be taken up and a large number of relatively long risers and mooring lines may be supported. By placing the buoy substantially completely within the recess, it can be protected during use in artic conditions against ice floes and allows the very large radial mooring forces to be transferred directly to the lower radial bearing of the turret at the same level without a moment being created between the connection point of the mooring lines and the flexible elements.

[0013] It is noted that a mooring construction is known from U.S. Pat. No. 5,339,760, in which the mooring lines are connected to a lower end of a mooring buoy which projects for a larger part from the receiving recess. Near the upper part a flexible element is provided between the mooring buoy and recess. Due to the large distance between the connection point of the anchor lines and the flexible elements, a moment is created on the buoy by the radial forces and shear forces are acting on the flexible compressible elements. Hereby the known to design is not capable of withstanding large radial forces.

[0014] Preferably, the sharp angle of the flexible element according to the present invention is between 1° and 45°, more preferably between 5° and 15° with respect to the vertical. By this substantially vertical orientation of the flexible element, large radial forces can be handled without the forces becoming problematic in the flexible element.

[0015] Preferably, the turret wall comprises a receiving surface which is attached to a radial bearing surface, such as a slide plate, that can be engaged with a second radial bearing surface on the hull of the vessel. In this way forces are directly transmitted from the turret to the vessel via the slide bearing thus reducing deformations of the lower part of the turret.

[0016] The buoy may comprise a ballastable body located below the lower draft level of the vessel when the buoy is coupled to the vessel. After coupling to the vessel, the buoy can be ballasted such that it exerts little or no upwards force and variation in draft level of the vessel will not result in changes of the upward force exerted by the buoy on the turret.

[0017] An embodiment of a floating mooring construction according to the present invention will be explained in detail with reference to the accompanying drawings in which:

[0018] FIG. 1 shows a vessel comprising a mooring construction according to the present invention, and

[0019] FIG. 2 shows a cross-sectional view of a floating production storage and offloading (FPSO) vessel connected to a subsea hydrocarbon structure via the mooring construction of the present invention.

[0020] FIG. 1 shows a mooring construction 1 according to the present invention comprising a vessel 2, such as FPSO, which is anchored to the seabed 3 via anchor chains or cables 4, 4′. The chains 4, 4′ are attached to mooring buoy 8 that is coupled to a turret 5 which is rotatably supported in the hull of the vessel via upper axial-radial bearings 6 that are connected to a stiff bearing box construction 7. The bearing box construction 7 isolates the deformations of the hull of the vessel from the axial-radial bearings 6. The mooring buoy 8 is seated in a recess or cavity 9, formed by the lower part of the turret 5. The anchor chains 4, 4′ extend through a chain hawse and tubes 10 and end at a chain stopper 11. The flow lines 13 are connected to a subsea hydrocarbon source on the seabed 3 and extend through passageways 14 in the buoy 8. At the upper end, the flow line 13 comprises valves and a fluid connector 15 for attaching to product piping on the vessel. The fluid connector 15 is located above the lower draft level 17 of the vessel.

[0021] Along its center line 21, the buoy 8 is connected to a pull in hawser in the form of chain 22 that is attached to a winch 24 that may for instance exert a force on the buoy 8 of 2000 tons.

[0022] The buoy 8 is at his upper end provided with a sealing element 40 along its perimeter, which may be an inflatable sealing element for closing off the space between the wall of the recess 9 and the outer surface of the buoy 8. Thereby, water may be drained from space 41 such that the fluid connectors 15 are located in a dry area, without water ingressing into the dry area 41 from the space between the buoy 8 and the wall of the recess 9 below the seal 40, which during use will be filled with water.

[0023] At its lower part the buoy 8 is provided with a circumferential groove 26 in which a flexible compression element 27 is seated. The flexible compression element 27 may for instance be any elastomeric pad made of rubber combined with stainless steel reinforcing plates. The flexible pad 27 makes an angle a of between 5° and 15° with respect to the vertical such that upon pulling in of the buoy 8 by the pull in chain 22, the flexible pad 27 is compressed against a receiving surface 29 extending at substantially the same angle &agr; to the vertical as the flexible pad 27.

[0024] The receiving surface 29 is part of a box construction 30 on the lower part of the turret 5 comprising a first radial bearing surface 32 and a second radial bearing surface 33 connected to radial bearing element 34 that is connected to the hull 35 of the vessel. Large radial mooring forces are transmitted via the flexible pads 27 and the slide pads 32, 33 from the relatively large diameter buoy 8 to the hull of the vessel thus limiting deformations of the lower part of the turret 5.

[0025] The diameter of the buoy 8 and the cavity 9 near their lower ends may for instance be 15 m and is larger than the diameter of the upper part 42 of the turret 5, which diameter may for instance be 5-6 m.

[0026] The relatively large buoy 8 comprises a ballastable compartment 36 which may after connection be flooded such that the buoy 8 does not contribute to an upwards force on the vessel during loading.

[0027] Upon connection of the buoy 8, the chain 22 is tensioned by the winch 24 until the lower part of the buoy is wedged into the recess 9 by compression of the flexible pads 27. Thereafter, a locking mechanism comprising a hydraulic ram 37 and clamping fingers 38 may be actuated to lock the upper buoy part in place. The chain 22 maintains its tensioning force until the locking mechanism is in place.

[0028] Even though the invention has been described with reference to an internal turret 5 located within the hull 35 of the vessel, the mooring construction can also be used in an external turret, mounted for instance in a yoke at the bow of the vessel. Furthermore, the compression element 27 may be placed directly on the outer wall of the buoy 8 or on the wall of the recess 9 or at on both walls, and may cover a larger length along said walls.

Claims

1. Floating mooring construction (1) comprising a vessel (2) having a hull (35) and having a cavity (9) for receiving a mooring buoy (8), the mooring buoy (8) carrying at least one flowline (13) and anchor lines (4, 4′, connected to the buoy via chain guides (10), and a pulling device (22, 24) connected to the mooring buoy for pulling the buoy towards the vessel, a flexible element (27) being comprised between a wall of the recess (9) and the buoy, characterised in that, the vessel comprises a turret (5) around which the vessel can weathervane, the recess (9) being provided in the lower part of the turret (5), wherein the buoy (8) or the recess (9) comprises near its lower end the flexible element (27), the outer surface of which includes a sharp angle (&agr;) with a centre line (21) of the buoy, the buoy being attachable to the turret by the pulling device (22, 24) pulling the buoy into the cavity (9) and compressing the flexible element (27) against the wall of the recess (9), the lower end of the buoy being located at or in the proximity of the lower end of the recess (9) and the anchor lines (4, 4′) being with their upper ends fixed to the buoy (8).

2. Floating mooring constructive (1) according to claim 1, wherein the flexible element (27) is seated in a an annular groove (26) on the recess (9) or on the buoy (8).

3. Floating mooring construction according to any of the preceding clams, wherein the diameter of the recess (9) and of the buoy (8) near their lower ends is larger than the diameter of a upper part (43) of the turret.

4. Floating mooring construction (1) according to claim 1, 2 or 3, characterised in that, the turret wall comprises a receiving surface (29) for engaging the flexible element (27), the receiving surface (29) making a sharp angle with the centre line (21) of the turret, a first radial bearing surface (32) being connected to the receiving surface for engaging with a second radial bearing surface (33) on the hull (35) of the vessel.

5. Floating mooring construction (1) according to claim 1, 2, 3 or 4, wherein the buoy (8) comprises at least one passage (14), the flowline (13) extending trough the passage (14) and having a fluid connector (15) at its end which is connectable to a fluid connector on the vessel.

6. Floating mooring device according to claim 5, wherein the buoy (18) or the recess (9) comprises near its upper end a seal (40) which can sealingly close the upper perimeter of the buoy against the wall of the recess (9) to seal off a chamber (41) around the fluid connectors (15) which can be drained to form a dry space.

7. Floating mooring construction (1) according to any of the preceding claims, characterised in that, the buoy (8) comprises a ballastable buoyant body (36) located below a lower draft level (17) of the vessel when the buoy is coupled to the vessel.

8. Floating mooring construction (1) according to any of the preceding claims, characterised in that, the flowline (13), at least at the position of the buoy (8), is flexible.